The art of carrying drill cuttings away from the drill bit and up to the surface in the course of penetrating geologic deposits is well known. Drilling muds, water, compressed air and other circulating fluids are used depending on local conditions. Also well known is the art of sampling a deposit after mining by intermittently passing a container or open chute through the falling stream as it comes off the end of a conveyor belt. But the art of sampling drill cuttings as they emerge from the drilled hole is, until now, unknown.
Sampling, as the term is used in the coal industry, means to obtain a reasonable amount of material which is representative of some larger amount of that material. To be considered "reasonable", a sample must be of such weight as to be handled conveniently in the field and at the laboratory. To be considered "representative", a sample must contain proportionate amounts of all constituents found in the larger amount of material from which it was taken. In the case of coal which has been partially crushed, (such as cuttings from blast holes), it is well known that "proportionate amounts" of constituents can only be present in the sample when the size distribution of the particles in the sample is identical to the size distribution in the large amount of material from which the sample was taken. Size distribution is important because coarse coal lumps are often of substantially different quality (sulfur content, heating value, ash content, etc.) than the fine dust, even though both came from the same coal deposit. Furthermore, it is recognized by knowledgeable coal sampling personnel that sample integrity is violated when size segregation occurs, and that rehomogenization can only be accomplished through considerable effort such as lengthy blending in a suitable apparatus.
In light of the above, a blast hole cutting sample must be small enough to be easily handled, and must contain the same natural particle size distribution as the total amount of cuttings produced by drilling the hole.
With the top of the coal deposit exposed, blast hole locations are marked by any convenient method. A blast hole drill rig is positioned at the site of the first hole and begins drilling. Compressed air is forced down through a hollow pipe which has a drill bit attached to its lower end. Holes in the drill bit permit the passage of air out through the bit. Since the drill bit diameter is somewhat larger than the pipe diameter an annular space is created between the outside of the pipe and the inside of the hole. As the pipe and the drill bit are rotated in the hole, cuttings are produced and carried upward. When they reach the surface they can simply be blown out of the hole and allowed to fall wherever they might, or they can be deflected away from the hole and then blown out onto the ground, or they can be first deflected away from the hole and then some method can be employed to collect them.
Hole diameters can vary from five or six inches to nine or ten inches, and coal thicknesses can vary from five feet to over fifty feet (and occasionally 75 to 100 feet). Therefore, the total weight of cuttings from a single hole can vary from about 100 pounds to nearly a ton or more.
The number of holes in a typical blast hole pattern might be 50 to 100 at some mines and might be several hundred at others. New blast hole patterns are sometimes drilled as often as every day or sometimes only once each week. Depending on the specifics of the mining operation and the coal deposit, there can be from five to more than one hundred tons of drill cuttings produced every week.
Clearly, all the drill cuttings produced at a mine cannot practically be collected and sent to a laboratory for testing. Some type of sampling must be employed to reduce the sheer volume of material. The most common method is to blow the cuttings onto the ground, then use a shovel or some type of partitioning device to divide each pile, or some pattern of piles, into smaller portions which are then sent to the laboratory. This method has the obvious drawback of failing to include in the sample the fine dust particles which are lost to the environment during drilling. These dust particles are often of different quality from the coarse chips and can have a substantial effect on apparent overall quality. Variations of this method have been tried at almost every surface coal mine and have met with only limited success in isolated cases.
Another approach to sampling is to collect all the cuttings from a hole using either a filter type bag house or a cyclone type separation system, or some combination of these. Because of the way these devices operate, the coarse material is removed first (typically by gravity) and the fine dust is removed in a separate step. This intentional separation according to size or weight allows these systems to work very efficiently as dust collectors and air purifiers, but renders them nearly useless as sample collectors. Good sampling practice requires not only that the size distribution in the sample be the same as the material that was sampled, but also that the size distribution in the sample not be reconstructed from its separate size constituents. A further drawback of the bag house/cyclone equipment is, again, the sheer volume of cuttings since this method collects all of the material produced by drilling the hole and makes no provision for reducing the volume of sample. This method also suffers from the high initial cost of auxiliary equipment including a truck or trailer on which to mount it, and substantial maintenance costs for replacement filters, motors, fans, etc. As with the previous method, the bag house/cyclone method has been tried at a number of mines, but is not being used anywhere as a sampling procedure.
Those concerned with these and other problems recognize the need for an improved chip sampler.